This invention relates to a method and apparatus for molding an optical element in which a resin layer is integrally molded on the surface of a glass member.
In the prior art, a method is known in which a thin film comprising a resin material is molded on the surface of a glass member to form a lens having an aspherical shape, which is difficult to work when the material is glass. The lens molded by this method generally is referred to as a replica lens.
As shown in FIG. 1, the method of molding a replica lens in the prior art includes placing a glass member 50, which has been worked into a spherical shape having a curvature approximating a desired aspherical shape, upon a mold member 52 having a molding surface 52a of the desired aspherical shape, filling a space 54 between a surface 50a of the glass member 50 and the molding surface 52a of the mold member 52 with a liquified resin and causing the resin to harden, thereby forming a replica lens 55 having the desired aspherical shape.
In the case where a resin material is molded by a mold in the manner described above, the resin material contracts as it cures. More specifically, before curing occurs, the resin material is introduced so as to fill the space 54 between the molding surface 52a of the mold member and the surface of the glass member 50. After hardening, the resin contracts so that the volume thereof becomes smaller than the volume of the space 54 defined by the molding surface 52a and the surface of the glass member 50.
The following phenomena occur owing to this reduction in the volume of the resin material:
(a) The glass member 50 itself flexes under a pulling force from the contracted resin.
(b) The resin material peels off the surface of the glass member 50.
(c) The resin material peels off the molding surface 52a of the mold member 52.
If the glass member 50 itself flexes as set forth in (a) above, the molded aspherical shape will merely undergo a slight shift from the shape desired. However, if the resin material peels off the glass member 50 as set forth in (b) above, the result is a fatal defect. Further, in the case where the resin material peels off the molding surface 52a, as in (c) above, the entire resin material does not peel off uniformly but only partially. As a consequence, a difference in the degree of contraction occurs between the portion of the resin material which has peeled off and the portion still clinging to the molding surface 52a. This causes the surface of the resin to be molded into shape having a curvature which is discontinuous, with the borderline being the portion of the resin material which has peeled off and the portion still clinging to the molding surface.